Exposure device, image forming apparatus, exposure control method and computer-readable medium

ABSTRACT

An exposure device includes a plurality of light-emitting elements, a lighting driver, a first storage, a reader, a shading correction unit and a second storage. The lighting driver drives and lights up the light-emitting elements based on image data. The first storage stores light amount unevenness correction values of the respective light-emitting elements. The reader reads the light amount unevenness correction values stored in the first storage. The shading correction unit executes shading correction for the light amount unevenness correction values read by the reader. The second storage stores correction values obtained by having the shading correction unit to execute the shading correction for the light amount unevenness correction values. The lighting driver controls light power of the respective light-emitting elements based on the correction values stored in the second storage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 fromJapanese Patent Application No. 2009-41807 filed on Feb. 25, 2009.

BACKGROUND

The present invention relates to an exposure device, an image formingapparatus, an exposure control method and a computer-readable mediumstoring a program that causes a computer to execute exposure controlprocessing.

SUMMARY

According to an aspect of the invention, an exposure device includes aplurality of light-emitting elements, a lighting driver, a firststorage, a reader, a shading correction unit and a second storage. Thelighting driver drives and lights up the light-emitting elements basedon image data. The first storage stores light amount unevennesscorrection values of the respective light-emitting elements. The readerreads the light amount unevenness correction values stored in the firststorage. The shading correction unit executes shading correction for thelight amount unevenness correction values read by the reader. The secondstorage stores correction values obtained by having the shadingcorrection unit to execute the shading correction for the light amountunevenness correction values. The lighting driver controls light powerof the respective light-emitting elements based on the correction valuesstored in the second storage.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will be described in detail basedon the accompanying drawings, wherein

FIG. 1 is a view showing the entire configuration of an image formingapparatus according to an exemplary embodiment of the present invention;

FIG. 2 is a sectional view showing the configuration of an LED printhead of the image forming apparatus according to one exemplaryembodiment of the present invention;

FIG. 3 is a plan view of an LED array 23, having plural LED chipsarranged therein, of the image forming apparatus according to oneexemplary embodiment of the present invention;

FIG. 4 is a circuit diagram showing a light-emitting element arraydriving unit in the LED print head, for which a self-scanning LED isadopted, of the image forming apparatus according to one exemplaryembodiment of the present invention;

FIG. 5 is a circuit diagram showing the light-emitting element arraydriving unit of the image forming apparatus according to one exemplaryembodiment of the present invention;

FIG. 6 is a timing chart of operations of respective parts of thelight-emitting element array of the image forming apparatus according toone exemplary embodiment of the present invention;

FIG. 7 is a view showing current flows in a level shift circuit when atransfer signal CK1R is turned from a default level to an L level in theimage forming apparatus according to one exemplary embodiment of thepresent invention;

FIG. 8 is a view showing current flows immediately after the transfersignal CKS is turned to a H level and CK1C is turned to an L level inthe image forming apparatus according to one exemplary embodiment of thepresent invention;

FIG. 9 is a view showing potentials of respective parts in a steadystate where a thyristor S1 is completely turned on, in the image formingapparatus according to one exemplary embodiment of the presentinvention;

FIG. 10 is a view showing a state, where gate current flows through athyristor S2, in the image forming apparatus according to one exemplaryembodiment of the present invention;

FIG. 11 is a block diagram of a control system of the LED array in theimage forming apparatus according to one exemplary embodiment of thepresent invention;

FIG. 12 is a circuit diagram of circuits with focusing on the drive unitin the image forming apparatus according to one exemplary embodiment ofthe present invention;

FIG. 13 is a schematic view showing the LED array in the image formingapparatus according to one exemplary embodiment of the presentinvention;

FIG. 14 is an enlarged plan view showing LED chips in the image formingapparatus according to one exemplary embodiment of the presentinvention;

FIG. 15 is a circuit diagram showing a shading correction calculationcircuit in the image forming apparatus according to one exemplaryembodiment of the present invention;

FIG. 16 is a graph showing correction of light amount unevennesscorrection values based on a correction coefficient 1 in the imageforming apparatus according to one exemplary embodiment of the presentinvention;

FIG. 17 is a circuit diagram showing circuits with focusing on the driveunit in the image forming apparatus according to one exemplaryembodiment of the present invention; and

FIG. 18 is a circuit diagram showing circuits with focusing on the driveunit of a related art.

DETAILED DESCRIPTION

Hereinafter, exemplary embodiments of the present invention of theinvention will be described.

FIG. 1 is a view showing the entire configuration of an image formingapparatus according to an exemplary embodiment of the present invention.

The image forming apparatus is able to form a color image on a printingmedium by a tandem type electrophotography system. The image formingapparatus is configured so that four drum-shaped photosensitive bodies1A, 1B, 1C and 1D are arranged around an intermediate transfer belt 7.Various types of devices and units to form images by theelectrophotography process are disposed around the photosensitive bodies1A, 1B, 1C and 1D, respectively. Since the configurations of thesedevise and units are common to the photosensitive bodies 1A, 1B, 1C and1D, herein, description is given of devices and units around thephotosensitive body 1A as representative. That is, a charger 2A, a printhead 3A, a developing device 4A, a cleaner 5A, and a charge neutralizer6A are arranged around the photosensitive body 1A. A toner image isformed on the photosensitive body 1A with a yellow (Y) developing agent(also, in the following description, the photosensitive bodies 1A, 1B,1C and 1D may be collectively referred to as the “photosensitive body”1, and this is the same as for the charger 2A, the print head 3A, thedeveloping device 4A, the cleaner 5A, and the charge neutralizer 6A).Similarly, toner images of magenta (M), cyan (C) and black (K) areformed on the photosensitive bodies 1B, 1C and 1D, respectively. Therespective toner images are stacked on each other and transferred ontothe intermediate transfer belt 7 while matching their positions based ondetection signals of a registration sensor 8, and all the toner imagesare collectively transferred onto a recording sheet 9. The recordingsheet 9 is conveyed to a fixing device 11 by means of a sheet conveyancebelt 10. The fixing device 11 fixes the toner images on the recordingsheet 9 (an example of a printing medium), thereby forming a colorimage.

Since, in such a tandem type color image forming apparatus, imageforming units of respective colors Y, M, C and K are independentlyarranged, it may be required to downsize the respective units.Therefore, it may demanded for the print head that a space occupancyratio around the photosensitive body circumference is downsized to asminimum extent as possible. An LED print head may be adopted, which usesan LED array in which a large number of light-emitting diodes (LEDs) (anexample of light-emitting elements) are arranged.

In the following description, detailed description is given on anexposure device for exposing a surface of the photosensitive body 1using the print head 3A.

FIG. 2 is a sectional view showing the configuration of an LED printhead.

The LED print head 20 is a light-emitting element for exposure of thephotosensitive body and is provided on the print head 3. The LED printhead 20 is provided with a housing 21 serving as a supporting body, aprinted circuit board 22 having a light-emitting element array driver 50(which will be described later) mounted thereon, an LED array 23 foremitting exposure light, a SELFOC® lens array (SELFOC lens is aregistered trademark of Nippon Sheet Glass Co., Ltd.) for focusing lightfrom the LED array 23 onto the surface of the photosensitive drum 1, aSELFOC lens array holder 25 for supporting the SELFOC lens array 24 andshielding the LED array 23 from the outside, and a leaf spring 26 forpressing the housing 21 in the SELFOC lens array 24 direction.

The housing 21 is formed of an aluminum or stainless steel block or madeof an aluminum or stainless steel sheet material, and supports theprinted circuit board 22 and the LED array 23. Also, the SELFOC lensarray holder 25 supports the housing 21 and the SELFOC lens array 23,and is configured so that the light-emitting point of the LED array 23is aligned with the focal point of the SELFOC lens array 24. Further,the SELFOC lens array holder 25 is disposed so as to closely seal theLED array 23. Therefore, no foreign substances such as dust are adheredto the LED array 23 from the outside. On the other hand, the leaf spring26 presses in the direction of the SELFOC lens array 24 via the housing21 so as to maintain the positional relationship between the LED array23 and the SELFOC lens array 24.

The LED print head 20 is configured so as to be movable in an opticalaxis direction of the SELFOC lens array 24 by an adjustment screw (notillustrated), and is adjusted so that an image formation position (thefocal point) of the SELFOC lens array 24 is located on the surface ofthe photosensitive drum 1.

In the LED array 23, as described later, plural LED chips 40 areaccurately arranged on a chip substrate to form a row and to be parallelto a shaft direction of the photosensitive drum 1. In the SELFOC lensarray 24, self-converging fibers are accurately arranged to form a rowand to be parallel to the shaft direction of the photosensitive drum 1.And, light from the LED array 23 is focused on the surface of thephotosensitive drum 1, and a latent image is formed thereon.

FIG. 3 is a plan view of the LED array 23 having plural LED chips 40arranged therein.

In the LED array 23, 58 LED chips 40 (C1 through C58) are accuratelyarranged to form a row and to be parallel to the shaft direction of thephotosensitive drum 1. The respective LED chips 40 are arrayed in azigzag manner. And, in the LED print head 20, 128 LEDs are incorporatedin each of the LED chips 40. In addition, the LED array 23 is providedwith a driver 41 to drive the LED chips 40. Further, the LED array 23 isprovided with a power circuit 61 to stabilize an output voltage, anEEPROM 62 to store light amount correction value data of the respectiveLEDs which constitute the LED chip 40, and a harness 63 for transmittingand receiving signals between the LED array 23 and an image formingapparatus main body.

Self-scanning LEDs are adopted in the LED print head 20. Theself-scanning LED adopts a thyristor structure as a portion equivalentto a switch that selectively turns on and off a light-emitting point. Byadopting the thyristor structure, it becomes possible to arrange theswitching portion on the same chip as that of the light-emitting point,and turn-on timing and turn-off timing of the switch are selectivelycontrolled for lighting by two signal lines. The data line can be madecommon, and the wiring thereof is simplified.

FIG. 4 is a circuit diagram showing the light-emitting element arraydriver 50 in the LED print head 20 in which the self-scanning LEDs areadopted.

In FIG. 4, the light-emitting element array driver 50 is provided withthe LED chip 40 and the driver 41 to drive the LED chip 40. The LED chip40 includes “n” thyristors S1, S2, . . . Sn (in the figure, thethyristors are appropriately illustrated by equivalent circuits), “n”light-emitting diodes (LEDs) L1, L2, . . . Ln, and “n+1” diodes CR0,CR1, CR2, . . . CRn, etc. In addition, the driver 41 includes resistorsRS, R1B, R2B, RID, capacitors C1, C2 and a signal generation circuit 42,etc. Also, in FIG. 4, only some of the thyristors, the light-emittingdiodes, and the diodes, which are provided in the LED chip 40, areillustrated.

Hereinafter, description is given on a circuit configuration of the LEDchip 40 and the driver 41. Anode terminals A1 through An of therespective thyristors S1 through Sn are connected to the power line 12.A power voltage VDD (VDD=3.3V) is supplied to the power line 12. Cathodeterminals K1, K3, . . . of the thyristors having an odd number (A1, A3,. . . ) are connected to the signal generation circuit 42 via theresistor R1A. A level-shift circuit 43 in which a signal line having theresistor R1B connected thereto and a signal line having the capacitor C1connected thereto are branched in parallel to each other is connectedbetween the resistor R1A and the signal generation circuit 42.Furthermore, cathode terminals K2, K4, . . . of the thyristors having aneven number (S2, S4, . . . ) are connected to the signal generationcircuit 42 via the resistor R2A. A level-shift circuit 44 in which asignal line having the resistor R2B connected thereto and a signal linehaving the capacitor C2 connected thereto are branched in parallel toeach other is connected between the resistor R2A and the signalgeneration circuit 42.

On the other hand, gate terminals G1 through Gn of the respectivethyristors S1 through Sn are connected to a power line 16 via resistorsR1 through Rn which are provided so as to correspond to the respectivethyristors S1 through Sn, respectively. In addition, the power line 16is grounded (GND).

The gate terminals G1 through Gn of the thyristors S1 through Sn are,respectively, connected to the gate terminals of the light-emittingdiodes L1 through Ln which are provided so as to correspond to therespective thyristors S1 through Sn.

Further, anode terminals of the diodes CR1 through CRn are connected tothe gate terminals G1 through Gn of the respective thyristors S1 throughSn. Cathode terminals of the diodes CR1 through CRn are, respectively,connected to the gate terminals of the next stage. That is, therespective diodes CR1 through CRn are connected to each other in series.

The anode terminal of the diode CR1 is connected to the cathode terminalof the diode CR0, and the anode terminal of the diode CR0 is connectedto the signal generation circuit 42 via the resistor RS. Further, thecathode terminals of the light-emitting diodes L1 through Ln areconnected to the signal generation circuit 42 via the resistor RID.Still further, the light-emitting diodes L1 through Ln are composed ofAlGaAsP or GaAsP as an example, and its band gap is approximately 1.5V.

FIG. 5 is a circuit diagram showing the light-emitting element arraydriver 50.

FIG. 5 shows the configuration of recording on an A3-sized recordingsheet at 600 dpi (dot per inch) and driving a 7424-dot LED element. Thatis, the LED print head 20 according to this exemplary embodiment hasfifty eight LED chips 40, each of which is composed of 128 dots.

In FIG. 5, ID that is an LED lighting signal is provided for each LEDchip 40, and 58 IDs are arranged in total. Also, each of the transfersignals CK1, CK2, CKS drive 9 or 10 chips. Six sets of the transfersignals CK1, CK2, CKS are arranged in total. The level shift circuits 43and 44 (see FIG. 4) are provided for each of the sets. With thisconfiguration, the drive capacity for each of the transfer signals CK1,CK2 and CKS is reduced, and all the LED chips 40 are driven in astabilized state at a low voltage.

Self-scanning LEDs are adopted in the LED print head 20. Theself-scanning LEDs employ the thyristor structure as a portioncorresponding to a switch that selectively turns on and turns off thelight-emitting points. By using the thyristor structure, the switchingportions are disposed on the same chip as the light-emitting points. Inaddition, since the turn-on timing and turn-off timing of the switch areselectively controlled for lighting by two signal lines, wherein thedata line can be made common, and the wiring thereof is simplified.

Next, description is given on operations of the light-emitting elementarray driver 50 shown in FIG. 4 with reference to a timing chart shownin FIG. 6. In FIG. 6, by showing the symbols, which are assigned to thesignal lines in FIG. 4, it is made clear to which signals of the circuitin FIG. 4 the respective signals correspond. Also, in the followingdescription, description is given on the case where four thyristors(n=4) are provided, as an example.

(1) First, in a default state, all the thyristors S1, S2, S3 and S4 areturned off since no current flows thereto (FIG. 6(1)).

(2) As the transfer signal CK1R is brought from the default state to anL level (FIG. 6(2)), current flows through the level shift circuit 43 ina direction of an arrow as shown in FIG. 7, and a potential of thetransfer signal CK1 becomes GND. Since the potential of the transfersignal CK1 is 3.3V in this example, a potential difference between theboth ends of the capacitor C1 is 3.3V (VDD). In this case, as shown bythe dotted-line in the timing of FIG. 6(2), the transfer signal CKS maybe set to a H level.(3) Simultaneously therewith, if the transfer signal CKS is set to the Hlevel and the transfer signal CK1C is set to an L level (FIG. 6(3)), thepotential of the transfer signal CK1 becomes approximately −3.3V sinceelectric charge is accumulated in the capacitor C1. Also, the potentialof the gate G1 becomes φS potential−Vf=approximately 1.8V. Here, the φSpotential is approximately 3.3V, and Vf means a forward directionvoltage of the diode of AlGaAs and is approximately 1.5V. Further, φ1potential=G1 potential−Vf=0.3V is brought about. Therefore, a potentialdifference of approximately 3.7V is produced between the signal line φ1and the transfer signal CK1.

And, in this state, gate current of the thyristor S1 begins flowing inthe route of the gate G1→signal line φ1→transfer signal CK1 as shown inFIG. 8. At this time, a tri-state buffer B1R is turned into a highimpedance (Hi-Z), wherein reverse flow of the current is prevented.

After that, Tr2 is turned on by the gate current of the thyristor S1,and the base current of Tr1 (collector current of Tr2) is caused toflow, and Tr1 is turned on, thereby causing the thyristor S1 to startturning on, and the gate current to gradually rise. In line therewith,since current flows in the capacitor C1 of the level shift circuit 43,the potential of the transfer signal CK1 gradually rises.

(4) After a predetermined duration of time (that is, a time period inwhich the potential of the transfer signal CK1 is brought into thevicinity of GND) elapses, the tri-state buffer B1R of the signalgeneration circuit 42 is brought to an L level (FIG. 6(4)). If so, thepotential of the signal line φ1 rises, and the potential of the transfersignal CK1 rises in line with a rise in the potential of the gate G1.Further, in line therewith, current begins flowing to the resistor R1Bside of the level shift circuit 43. On the other hand, the currentflowing in the capacitor C1 of the level shift circuit 43 graduallydecreases in line with a rise in the potential of the transfer signalCK1.

Then, as the thyristor S1 is completely turned on and is brought into asteady state, the potentials of the respective signal lines become asshown in FIG. 9. That is, although current to keep the thyristor S1 in aturned-on state flows in the resistor R1B of the level shift circuit 43,no current flows in the capacitor C1. Further, the potential of thetransfer signal CK1 is CK1 potential=1.8−1.8×R1B/(R1A+R1B).

(5) The lighting signal ID is brought to an L level with the thyristorS1 being completely turned on (FIG. 6(5)). At this time, since the gateG1 potential is larger than the gate G2 potential (Gate G1potential−Gate G2 potential=1.8V), the LED L1 of the thyristor structureis turned on earlier and is lit. In line with lighting of the LED L1,the potential of the signal line φ1 rises to cause signal line φ1potential=gate G2 potential=1.8V to be brought about. Therefore, theLEDs including LED L2 and subsequent LEDs will not be turned on. Thatis, among the LEDs L1, L2, L3, L4 . . . , only the LED having thehighest gate voltage is turned on (lit).(6) Next, as the transfer signal CK2R is set to an L level (FIG. 6(6)),current flows as in the case of FIG. 6(2), and a voltage is generatedbetween the both ends of the capacitor C2 of the level shift circuit 44.In a steady state immediately before the step of FIG. 6(6) is finished,since the gate G2 potential is 1.8V, the voltage values at therespective points slightly differ from those in the case of FIG. 6(2).However, no influence is brought about. The reason is as describedbelow. The potential of the signal line φ2 is 0.3V or so (=Gate G2potential−Vf=1.8V−1.5V) in a steady state immediately before the step ofFIG. 6(6) is finished. Therefore, the gate current flows to thethyristor S2 in the dotted line direction as shown in FIG. 10. However,since this gate current is only slight, the thyristor S2 is not turnedon. In this case, the transfer signal CK2 potential is roughly 0.15V orso (=CK2 potential=0.3−0.3×R2B/(R2A+R2B).(7) If the transfer signal CK2C is set to an L level in this state (FIG.6(7)), the thyristor switch S2 is turned on.(8) Then, if the transfer signals CK1C and CK1R are simultaneously setto the H level (FIG. 6(8)), the thyristor switch S1 is turned off, andthe gate G1 potential gradually falls by discharge through the resistorR1. At this time, the gate G2 of the thyristor switch S2 becomes 3.3V,and is completely turned on. Therefore, by bringing lighting signal IDterminals corresponding to image data to L level/H level, the LED L2 canbe brought into lighting and non-lighting. Also, in this case, since thegate G1 potential has already been lower than the gate G2 potential, theLED L1 will not be turned on.

Thus, according to the light-emitting element array driver 50, since theON state of the thyristor switches of the thyristors S1, S2, . . . Sncan be changed by alternately driving the transfer signals CK1 and CK2,the LEDs L1, L2, . . . Ln are selectively controlled for lighting ornon-lighting through time sharing.

FIG. 11 is a block diagram of a control system of the LED array 23 thatbecomes an exposure device.

In this control system, a ROM 103, a RAM 104, a communication interface(I/F) 105 are connected to a CPU 101 that collectively controlsrespective parts. The ROM 103 stores various types of control programs102 executed by a CPU 101 and fixed data. The RAM 104 serves a work areaof the CPU 101. The communications interface (I/F) 105 executescommunications with the LED array 23. The control programs 102 may bestored in advance at the beginning of production of the image formingapparatus. Alternatively, the control programs may be set up in astorage device by reading the same later from a storage medium havingthe control programs 102 stored therein or downloading the same viacommunications tool such as the Internet.

FIG. 12 is a circuit diagram showing the circuits with focusing on thedriver 41.

Individual LEDs of the LED chip 40 are not uniform and are uneven in thelight-emitting amount thereof. Thus, it is necessary to make it uniformby correcting unevenness (mura) in the amount of light. In this case,light amount unevenness correction data is stored in EEPROM 62 of theLED array 23. The light amount unevenness correction data stored in theEEPROM 62 is all read out into a light amount unevenness correctionmemory 111 implemented by a RAM.

The CPU 101 outputs a reset signal to the driver 41 whenever the mainpower of the image forming apparatus is turned on. Also, the CPU 101outputs a reading signal TRG to the driver 41 when it is detected by asensor (not illustrated) inside the image forming apparatus that thelight amount of the LED array 23 has partially fallen. An OR circuit 141outputs a Trg signal to the EEPROM access control circuit 142 when thereset signal or the TRG signal is output, and the EEPROM access controlcircuit 142 controls so that light amount unevenness correction valuesare read from the EEPROM 62 and are input into the light amountunevenness correction value memory 111.

A lighting pulse number calculation circuit 112 calculates alight-emitting time of LED per dot image based on the light amountunevenness correction data stored in the light amount unevennesscorrection value memory 111. In this exemplary embodiment, thelight-emitting power of the respective LEDs is controlled by controllingthe light-emitting time. Raster image data (image data in order from anedge of a printing image) generated by an image data generation circuit121 is converted to lighting order image data by an image datare-arrangement conversion circuit 113 of the driver 41, and istransmitted to a lighting signal generation circuit 114.

FIG. 13 is a schematic view of the LED array 23.

The LED array 23 in FIG. 13 does not accurately express the array of theLED chips 40 as in FIG. 3, but is shown in a single row extending in theleft and right direction for convenience. Herein, a relationship betweenthe LED chip 40 of the LED array 23 and a sheet feeding direction of asheet P is shown. In raster image data, image data of respective dotsare arranged in order of an arrow that is shown as an image datareceiving order.

FIG. 14 is an enlarged plan view of the LED chip 40.

Plural LEDs are arranged in a row in the respective LED chips 40, andare shown with reference numeral 131. In this example, the respectiveLEDs 131 of the light side LED chip 40 are lit in order from the leftend thereof. The respective LEDs 131 of the right side LED chip 40 inparallel thereto are lit in order from the right end thereof. In thiscase, dots formed by the respective LEDs 131 are shown with referencesymbol “d.” The lighting order image data is such data that respectivedots of image data are arranged in order matched to the lighting orderof such respective LED chips 40.

Returning to FIG. 12, the lighting signal generation circuit 114 drivesthe respective LED chips 40 based on the lighting order image data. Inthis case, the lighting time of each dot is determined by the lightingpulse number calculated by the lighting pulse number calculation circuit112, thereby correcting unevenness in the amount of the light.

On the other hand, distances between the respective LEDs of the LED chip40 and the photosensitive bodies 1A, 1B, 1C and 1D (hereinafter, may bereferred to as the photosensitive body 1A as the representative) vary inpositions on the photosensitive body 1A due to assembling errors ofcomponents. Therefore, there may be cases where a formed image may bepartially thin or pale. Also, a toner remaining on the photosensitivebody 1A after an image is formed is scraped off by a blade. Therefore,abrasion unevenness may occur on the surface thereof, and unevenness(mura) in the sensitivity may be brought about even if the same lightamount is received from the LEDs.

Accordingly, in order to correct image unevenness (mura) brought by suchassembling errors of components and unevenness (mura) in sensitivity ofthe photosensitive body, shading correction is executed in thisexemplary embodiment.

That is, the CPU 101 stores shading correction coefficients for shadingcorrection in a register 151, and a shading correction calculationcircuit 152 corrects the light amount unevenness correction values,which are stored in the EEPROM 62, based on the shading correctioncoefficients, and the corrected light amount unevenness correctionvalues are stored in the light amount unevenness correction value memory111.

FIG. 15 is a circuit diagram of the shading correction calculationcircuit 152.

The shading correction coefficients used in this example include fivecorrection coefficients of correction_(K0), correction_(K1),correction_(K2), correction_(K3) and correction_(K4). A selector 161 ofa correction coefficient calculation section 169 selects one of the fiveshading correction coefficients based on a coefficient selection signal.An adder 162 adds an output value of a D flip flop 164 to the shadingcorrection coefficient thus selected. A selector 163 outputs one of thecorrection_(int) being an initial value of the corrected light amountunevenness correction value (the light amount unevenness correctionvalue subjected to the shading correction) and the value output by theadder 162. In this case, the correction_(int) is selected as an initialvalue, and thereafter the value output by the adder 162 is selected. TheD flip flop 164 holds the selected value and outputs it to a correctionvalue calculation circuit 165 as a correction coefficient 1.

A correction coefficient storage section 166 stores correction data ofrespective LEDs (for example, 256 LEDs) for each LED chip 40. This iscorrection data to correct a light amount unevenness which is generatedin units of the LED chips 40 due to deterioration of the product. Thecorrection data stored in the correction coefficient storage section 166are output to the correction value calculation circuit 165 as acorrection coefficient 2.

The correction value calculation circuit 165 corrects the light amountunevenness correction values based on the correction coefficients 1 and2, and stores the corrected light amount unevenness correction values inthe light amount unevenness correction value memory 111.

FIG. 16 is a graph showing correction of the light amount unevennesscorrection values based on the correction coefficient 1.

The horizontal axis of the graph indicates the respective LEDs of theLED array 20, and the vertical axis of the graph indicates the lightamount unevenness correction values.

In this case, for a predetermined range of the LEDs, the same shadingcorrection coefficient is commonly used. In this example, thecorrection_(K0) of the shading correction coefficient is applied in theinterval from 0 through 2047 dots, and the other correction_(K1),correction_(K2), correction_(K3) and correction_(K4) are applied inrespective fixed intervals. Since the adder 162 always adds the sameshading correction coefficient to the light amount unevenness correctionvalues of a dot just prior to a current dot in the same interval, thelight amount unevenness correction value simply increases or simplydecreases at a fixed ratio in one interval. In this example, correctionis executed using the correction_(int) as the initial value so that thelight amount unevenness correction value becomes large at both end partsof the LED array 23 and that the light amount unevenness correctionvalue becomes small in the middle part of the LED array 23.

In the circuit described above, the shading correction calculationcircuit 152 corrects the light amount unevenness correction valuesthrough calculations. Hereinafter, description is given on the circuitconfiguration according to this exemplary embodiment, in which the loadof the calculation process in the shading correction calculation circuit152 is reduced.

FIG. 17 is a circuit diagram showing circuits with focusing on thedriver 41.

Since the plural LED chips 40 are provided, the driver 41 is alsoprovided with plural lighting signal generation circuits 114 and plurallighting pulse number calculation circuits 112.

The light amount unevenness correction values transmitted from theEEPROM 62 require 8 or more clocks for each of 8-bit data in the case ofserial protocol. If a communication rate is set to 5 Mhz or so at amaximum, it takes approximately 2 μsec that a light amount unevennesscorrection value of one LED is transmitted to the shading correctioncircuit 152.

It is assumed that an image forming process speed is 200 mm/sec, andthat a resolution in the sub-scanning direction is 1200 dpi. Also, it isassumed that there are sixty LED chips 40 each of which 105 μsec isassigned in an exposure time for one line, that, that two lightingsignal generation circuits 114 and two lighting pulse number calculationcircuits 112 are provided for each LED chip, that the simultaneous LEDlighting number is 120 dots, and that 256 LEDs are provided in each LEDchip 40. In this case, it is necessary that 120 pieces of data areprocessed during 105 μsec, and this process is executed 128 times.Therefore, it would be necessary to execute the processing per dotwithin approximately 800 ns (120 LEDs are simultaneously processed).

The reset signal is output from the CPU 101 when the main power sourceof the image forming apparatus is turned on. The light amount unevennesscorrection values are transmitted from the EEPROM 62 to the shadingcorrection calculation circuit 152. Then, the shading correctioncalculation circuit 152 may correct the light amount unevennesscorrection values one after another in the receiving order, and thecorrected light amount unevenness correction values are stored in thelight amount unevenness correction value memory 111.

In this manner, all the light amount unevenness correction values areonce corrected, and all the corrected light amount unevenness correctionvalues are stored in the light amount unevenness correction value memory111. Then, when an image is to be formed, each lighting pulse numbercalculation circuit 114 reads the light amount unevenness correctionvalue of a corresponding one of the dots which each circuit 114 is incharge of, from the light amount unevenness correction value memory 111,and calculates the lighting time of the corresponding one of the dots.

FIG. 18 is a circuit diagram of circuits with focusing on the driver 41according to a related art.

When the circuit of FIG. 18 is compared with the circuit according tothis exemplary embodiment of FIG. 17, the shading correction calculationcircuit 152 is not provided at a preceding stage of the light amountunevenness correction value memory 111 but is provided at a subsequentstage thereof. Also, the plural shading correction calculation units 152are provided so as to correspond to the respective lighting pulse numbercalculation circuits 112.

Therefore, light amount unevenness correction values for which shadingcorrection has not been performed are stored in the light amountunevenness correction value memory 111. When an image is to be formed,each shading correction calculation circuit 152 reads the light amountunevenness correction value from the light amount unevenness correctionvalue memory 111, calculates the light amount unevenness correctionvalue for which the shading correction has been executed, and transmitsit to the lighting pulse number calculation circuit 112.

In such a circuit configuration of the related art, calculation of thelight amount unevenness correction values for which shading correctionhas been executed is carried out when an image is formed. Therefore, itis necessary to calculate the light amount unevenness correction values,for which shading correction has been executed, corresponding to imagedata transmitted one after another. Therefore, the load of calculationprocesses is increased. Further, the respective shading correctioncalculation circuit 152 does not correct the light amount unevennesscorrection values transmitted from the EEPROM 62 one after another, butit is necessary for the shading correction calculation circuit 152 toselectively read only the light amount unevenness correction values,which will be required in the respective shading correction calculationcircuits 152. Therefore, the load of the calculation processes will befurther increased. In addition, since the plural shading correctioncalculation circuits 152 are provided so as to correspond to therespective lighting pulse number calculation circuits 112, the circuitconfiguration will also be large-sized.

The foregoing description of the exemplary embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Obviously, many modificationsand variations will be apparent to practitioners skilled in the art. Theembodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with the various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

1. An exposure device comprising: a plurality of light-emittingelements; a lighting driver that drives and lights up the light-emittingelements based on image data; a first storage that stores light amountunevenness correction values of the respective light-emitting elements;a reader that reads the light amount unevenness correction values storedin the first storage; a shading correction unit that executes shadingcorrection for the light amount unevenness correction values read by thereader; and a second storage that stores correction values obtained byhaving the shading correction unit to execute the shading correction forthe light amount unevenness correction values, wherein the lightingdriver controls light power of the respective light-emitting elementsbased on the correction values stored in the second storage, wherein theshading correction unit is provided at a preceding stage of the secondstorage, wherein the shading correction unit executes shading correctionfor the light amount unevenness correction values read by the reader byusing a first correction coefficient output from a correlationcoefficient calculation section and a second correction coefficientoutput from a correction coefficient storage section, and the secondcorrelation coefficient corrects at least one of the light amountevenness of the light-emitting elements and abrasion evenness on asurface of photosensitive bodies, wherein the correlation coefficientcalculation section comprises: a first selector that selects a firstintermediate selection signal from a plurality of correctioncoefficients based on a coefficient selection signal; a flip flop thatoutputs the second correction coefficient based on a second intermediatesignal; an adder that adds the output of the flip flop with the firstintermediate selection signal and outputs an added signal; and a secondselector that selects one of an initial correction value and the addedsignal and sends the second intermediate signal to the flip flop.
 2. Animage forming apparatus comprising: a photosensitive body; an exposuredevice that forms a latent image on the photosensitive body; and adeveloping device that develops the latent image, wherein the exposuredevice includes a plurality of light-emitting elements, a lightingdriver that drives and lights up the light-emitting elements based onimage data, a first storage that stores light amount unevennesscorrection values of the respective light-emitting elements, a readerthat reads the light amount unevenness correction values stored in thefirst storage, a shading correction unit that executes shadingcorrection for the light amount unevenness correction values read by thereader, and a second storage that stores correction values obtained byhaving the shading correction unit to execute the shading correction forthe light amount unevenness correction values, and the lighting drivercontrols light power of the respective light-emitting elements based onthe correction values stored in the second storage, wherein the shadingcorrection unit is provided at a preceding stage of the second storage,wherein the shading correction unit executes shading correction for thelight amount unevenness correction values read by the reader by using afirst correction coefficient output from a correlation coefficientcalculation section and a second correction coefficient output from acorrection coefficient storage section, and the second correlationcoefficient corrects at least one of the light amount evenness of thelight-emitting elements and abrasion evenness on a surface of thephotosensitive body, and wherein the correlation coefficient calculationsection comprises: a first selector that selects a first intermediateselection signal from a plurality of correction coefficients based on acoefficient selection signal; a flip flop that outputs the secondcorrection coefficient based on a second intermediate signal; an adderthat adds the output of the flip flop with the first intermediateselection signal and outputs an added signal; and a second selector thatselects one of an initial correction value and the added signal andsends the second intermediate signal to the flip flop.
 3. Acomputer-readable medium storing a program that causes a computer toexecute an exposure control process, wherein an exposure device includesa plurality of light-emitting elements, a lighting driver that drivesand lights up the light-emitting elements based on image data; a firststorage that stores light amount unevenness correction values of therespective light-emitting elements; a reader that reads the light amountunevenness correction values stored in the first storage; a shadingcorrection unit that executes shading correction for the light amountunevenness correction values read by the reader; and a second storagethat stores correction values obtained by having the shading correctionunit to execute the shading correction for the light amount unevennesscorrection values, the lighting driver controls light power of therespective light-emitting elements based on the correction values storedin the second storage, and the exposure control process comprises:controlling the exposure device to execute the shading correction forthe light amount unevenness correction values stored in the firststorage, and to store the correction values obtained by the executing ofthe shading correction in the second storage, wherein the shadingcorrection unit is provided at a preceding stage of the second storage,wherein the shading correction unit executes shading correction for thelight amount unevenness correction values read by the reader by using afirst correction coefficient output from a correlation coefficientcalculation section and a second correction coefficient output from acorrection coefficient storage section, and the second correlationcoefficient corrects at least one of the light amount evenness of thelight-emitting elements and abrasion evenness on a surface ofphotosensitive bodies, wherein the correlation coefficient calculationsection comprises: a first selector that selects a first intermediateselection signal from a plurality of correction coefficients based on acoefficient selection signal; a flip flop that outputs the secondcorrection coefficient based on a second intermediate signal; an adderthat adds the output of the flip flop with the first intermediateselection signal and outputs an added signal; and a second selector thatselects one of an initial correction value and the added signal andsends the second intermediate signal to the flip flop.